direct gain systems
The direct gain is the most common passive solar system in architecture.Direct gain is the collection and containment of radiant solar energy within the occupied space. Sunlight enters the home through south-facing windows and heats the room. It also strikes floors and walls, which absorb, store and reradiate heat to the home's interior. If heavy construction materials are used for these interior walls, their mass serves both to minimize drastic temperature changes from night to day and to store the heat energy for sunless periods. Thermal storage mass can consist of a concrete slab, water containers, an interior masonry wall (a chimney is ideal), and masonry that together provide the correct amount of total mass.With the direct gain system, the thermal storage mass may be thinner and more widely distributed in the living space than with other passive systems. This allows an even distribution of heat throughout the room or rooms, but requires some thought about how the living space will be used. Don't cover the thermal storage mass with carpet or other materials that will reduce its storage capacity. Select and arrange furnishings carefully so they don't interfere with solar collection, storage, and distribution.
Requirements of Direct Gain System 
  • Large south facing glazed (collector) area, with the living space exposed directly behind.
  • A floor and/or wall storage mass of significant dimension for solar exposure and for capacity.
  • A method for isolating the storage from exterior climate conditions.
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    1. Direct gain is the simplest approach and usually the most economical to build. With this system, sunlight enters the house through large areas of south-facing glass. It heats the floor and walls directly. Energy from the mass in floors and walls is released to the living space when the inside air temperature is lower than that of the mass. In this system, the actual living space is a solar collector, heat absorber and distribution system. South facing glass admits solar energy into the house where it strikes directly and indirectly thermal mass materials in the house such as masonry floors and walls. The direct gain system will utilize 60 - 75% of the sun's energy striking the windows.
    2. Clerestory windows and skylights are sometimes used to increase the amount of sunlight hitting the back area of walls or floors. They can help improve the performance of the direct gain system. Skylights, however, tend to create overheating problems in the summer and may leak if improperly installed. Clerestory windows in a direct gain system let sunlight strike the thermal mass on the back wall.
    3. The amount of south-facing glass and thermal storage mass should be balanced. If the windows collect more heat than the floor or walls can absorb, overheating occurs. Since the direct gain system is part of the living space, this can be uncomfortable for those living in the house.
     
     

     
    4. Shading is needed to reduce heat gain in the summer. Overhangs, awnings, trellises, louvers, solar screens, and movable insulation are some choices. Most designers recommend exterior shading rather than interior shading because exterior screens and other devices stop heat before it gets into the house.The overhang lets in the winter sun while shading southfacing glass in the summer.

     Advantages

    • It is comparatively low in cost to build, since no special room has to be added. The floor, walls, or even an inside-wall fireplace can serve as the storage mass. The solar elements are incorporated into the living space.
    • It provides direct heating. There is no need to transfer energy from one area to another. South-facing windows provide natural daylight and outdoor views. 
    • The number and size of south-facing windows can be adjusted to match the space you have for thermal mass. Clerestory windows can let sunlight fall directly on the back parts of floors or walls used as thermal mass
    Disadvantages of the Direct Gain System:
    • It can overheat if the windows and thermal mass are not balanced.
    • Large amounts of south-facing glass can cause problems with glare and privacy.
    • The thermal mass used for heat storage should not be covered by carpet or blocked by furnishings.
    • Furnishings and fabrics exposed to ultraviolet radiation from the sun can degrade or change color.

      • South-facing windows need summer shading and a nighttime insulative covering in winter. Nighttime insulation can be provided by exterior-mounted panels, interior draperies, shutters, pop-in panels, or other insulating window treatments.
    Thermal mass in the interior absorbs the sunlight and radiates the heat at night
    In a direct gain system, the thermal mass floors and walls are functional parts of the house. It is also possible to use water containers inside the house to store heat. However, it is more difficult to integrate water storage containers in the design of the house.

    The thermal mass will temper the intensity of the heat during the day by absorbing the heat. At night, the thermal mass radiates heat into the living space.
    Direct gain system rules of thumb 
    • A heat load analysis of the house should be conducted.
    • Do not exceed 6 inches of thickness in thermal mass materials.
    • Do not cover thermal mass floors with wall to wall carpeting; keep as bare as functionally and aesthetically possible.
    • Use a medium dark color for masonry floors; use light colors for other lightweight walls; thermal mass walls can be any color.
    • For every square foot of south glass, use 150 pounds of masonry or 4 gallons of water for thermal mass.
    • Fill the cavities of any concrete block used as thermal storage with concrete.
    • Use thermal mass at less thickness throughout the living space rather than a concentrated area of thicker mass.
    • The surface area of mass exposed to direct sunlight should be 9 times the area of the glazing.
    • Sun tempering is the use of direct gain without added thermal mass. For most homes, multiply the house square footage by 0.08 to determine the amount of south facing glass for sun tempering.