|Design Handbook on Passive Solar Heating and Natural Cooling (HABITAT, 1990, 162 p.)|
|V. Basic design principles and strategies|
The three systems for passive solar heating generally accepted today are direct gain, thermal storage walls (masonry or water), and the attached sunspace. These are described below. Further Information and design details can be found in the literature on passive solar design listed in the bibliography. The reader will also find that opinions about precise details will vary from author to author in much the same way that opinions vary about construction details. Passive solar design does not have just one correct solution. Correct solutions are as varied as the designer's imagination.
1. Direct gain
Direct-gain systems are those where the solar radiation enters the habitable spaces via equator-facing windows and is absorbed by the materials inside the space for later dissipation when the ambient temperature falls. This is by far the most commonly used system and in spatial design terms the most flexible because it can accommodate daylighting and views to the exterior as well as providing a source of heat. Whilst it is the easiest for the designer to work with, it does have far more limitations than some of the other systems.
Figure 54. An example of direct gain
To operate effectively, the area of the equator-facing direct-gain window is closely related to the area of thermal-storage materials within the space and the climate characteristics of the particular location. The thermal-storage area must be matched to the incoming solar radiation to maintain the diurnal temperature swing in the range of 5-6C for acceptable thermal comfort. Insufficient thermal storage could result in daytime overheating even in quite cold climates. In temperate climates, where heating loads are modest. this system should provide high levels of thermal comfort without any significant auxiliary heating. In very cold climates there is likely to be insufficient solar contribution due to the limitations of glass area and it may be more appropriate to use a combination of systems. Manual techniques for the design of appropriate window size and area of thermal-storage material are given in many publications such as the SLR-method. A computer-based calculation method, CHEETAH, is also available.
The equator-facing windows must be designed with appropriate shading and means of minimizing conductive heat loss as described earlier. Direct sunlight entering the space can be a problem in terms of fading of interior finishes and also glare for the occupants. The latter can be countered in most domestic situations by appropriate selection of surface finishes. Direct sunlight inside a room has many psychologically beneficial qualities which have been recognized down through the ages and so this should not be overlooked.
Roof-level glazing can provide a useful contribution to the equator-facing glass area provided its thermal resistance is adequate to minimize heat losses. In temperate climates and those locations where the summers are quite hot, it is important to avoid sloping glass. In any event, all glass must be well shaded to stop the entry of summer sun as discussed in more detail later.
The floor is most commonly the main termal storage medium in direct-gain systems because so often it is the most economical place to locate heavy masonry materials. Unless there is an adequate amount of thermal storage in the walls of the space (perhaps at least half the walls) the floor should be of a material with a high thermal admittance (see chapter 111) and have the least amount of thermally-resistive covering (i.e., carpets or cork tile). The surface should have a low reflectance to absorb as much of the incoming energy as possible. Where there is a significant thermal storage component in the walls. the floor may be more reflective the better too distribute the sun's energy to the other heat storage surfaces.
2. Thermal storage walls (Trombe wolfs)
Thermal storage wall systems usually comprise a dark-coloured heavy wall (masonry, concrete, mud brick etc.) erected in the solar aperture with a double-glass system mounted approximately 1 " (20-30mm) in front of the wall surface. The sun's radiation passes through the glass and is mostly absorbed by the dark surface of the wall (preferably of high absorbency and low emissivity). The amount of energy that is lost back through the glazing depends on the insulating properties of the glazing and the surface properties of the wall. Special surface coatings for thermal storage walls with low emissivity and high absorbency are manufactured In North America specifically for this application.
The solar energy absorbed into the wall raises its temperature slowly throughout the day. By night-time the heat has penetrated to the inside surface where it can radiate Into the room raising the environmental temperature so providing warmth to the occupants. The time taken for the heat to reach the inner surface depends on the thermal properties and the thickness of the wall material, typically about 10 hours per foot of thickness for masonry. The same design guidelines and computer-based design tools listed for direct-gain systems also provide for thermal storage walls.
In early examples a range of movable night insulation systems were employed but Balcomb now reports that experience has shown double-glazing and selective surface coatings on the outer face of the thermal storage wall to be more reliable and cost-effective. Current recommendations are that the air space between the glass and the wall surface should be well sealed. The earlier examples had various systems of venting either to the room being heated for winter or the outside to reject heat in summer.
Protection from solar radiation in the summer is best provided by some form of cover over the glazing. The protection that is appropriate for direct-gain windows may not be sufficient to keep out the diffuse- and ground-reflected forms of radiation. Unwanted solar heat in direct-gain heated spaces can be dissipated by natural ventilation whereas in thermal storage walls it will be absorbed into the walls to heat the room when not wanted.
In some cases water has been used as the thermal storage material. This technique of passive solar heating is similar to the thermal storage wall except for a number of key points. The thermal storage capacity of water Is approximately three times that of brickwork by volume (see table 8). Whilst it is inexpensive as a material it is both difficult and relatively expensive to store securely. The time-lag effect of water in containers is shorter than in masonry because it is a fluid: the transfer of energy is part by conduction and part by convection. Many examples built so far use water in vertical cylinders painted a dark colour or finished with a selective coating as described for the thermal storage walls. The cylinders can be placed with spaces between them to allow some solar radiation and daylight to penetrate directly into the space. The water-storage walls generally perform better than thermal storage walls because the convection mixing of the water results in lower surface temperatures and a more even distribution of energy. Most of the design tools that have been developed for use with thermal storage walls are also suitable for the design of water-based heat-storage systems.
Figure 55. A thermal-storage wall system
Figure 56. Sunspace enclosures
3. Sunspace enclosures
An attached sunspace system comprises an additional enclosure, usually with all surfaces glazed, attached to the equator-facing side of the building to be solar heated. The additional space so created can be used for various purposes that are suited to exposure to high levels of sunlight and a wider temperature range than aimed for inside the rest of the building. The built form of the sunspace may range from simple attachment to the face of the building to full integration into the building envelope. A number of sunspace configurations have been defined in the literature. for use in design tool analysis. They are essentially variations on a theme that have been defined for computer-modelling purposes. In reality, an attached sunspace is a room on the equator-facing side of a building with a significantly high glass area compared to the floor area (glass/floor ratio > 1).
The critical components of a sunspace solar-heating system are the floor (usually ground connected), the wall separating the sunspace from the remainder of the building to be heated and the area and nature of the glazing. The wall dividing the sunspace from the rest of the building is similar to the thermal storage wall described before. In this case the glass has been moved out to form an accessible space for other purposes. thus making the overall system more economically viable. The dividing wall can be either a solid mass, uninsulated, insulated on the inside, insulated on the outside or simply light-weight heavily insulated. If the wall is of uninsulated heavy materials then the same rules apply as with the thermal storage wall. If it is insulated then a venting system is required to transport the collected heat to the interior of the building. The use of thermal mass materials inside the sunspace will help to modify the diurnal temperature swing.
In all locations except where summers are very cool, it is important to provide adequate through ventilation to remove summer heat build-up (minimum vent area should be 10 per cent of the total glass area for mild summer climates and more for warm to hot summer climates). In most areas where summers are warm (mean temperatures within the comfort range) it will be necessary to shade the glass area fully to avoid overheating and degradation of the interior surfaces and fitments. The preferred design for such areas is with a fixed opaque roof and only the vertical surfaces glazed. Even in colder slimates there is likely to be problems with overheating in autumn as the sun is passing low in the sky and yet the temperatures are not commensurately lower. This can be overcome to some extent with suitable shading of the roof glass and the east and west glass, if any. The current trend is to build such enclosures as a visual element in an otherwise conventional building. Unfortunately, too often little thought is given to the protection of the glass areas from solar gain in summer and, as a result, many unsatisfactory buildings are being built.
In many applications an attached sunspace is used as a multi- level connecting space (lower floor with mezzanines above to the upper floors). In such cases the designer must be aware of the chimney effect of the must be aware of the chimney effect of the tall space as the air in the sunspace will be much warmer than the remainder of the house. The design should allow for the cooler air inside to flow down to the lower area and out to the sunspace. With large glass areas, overheating can be quite a serious problem without correct shading.