Passive solar heating is the simple concept of allowing solar gain into a building when heat is needed in the winter. The solar gain during the day is partially stored in thermal mass for later use during the night. The thermal mass (concrete, brick, adobe) will heat up during the day and then deliver heat to the space it is located in through two processes. Convection air currents across the thermal mass will heat the air in the room, and radiation from the warm thermal mass will increase the mean radiant temperature of the room. The increase in the mean radiant temperature of a room allows the thermostat controlling room air temperature to be set lower while still providing thermal comfort.
For passive solar to work it is necessary to maximize solar gain in the winter and minimize solar gain in the summer. South facing glass provides this balance (Figure 14.1). East and west facing glass has about three times more solar gain in summer than in winter. Horizontal glass has almost three and a half times more solar gain in summer than in winter. South facing glass has the opposite characteristic of having close to three times more solar gain in winter than in summer.
Before passive solar techniques are used to size south facing windows and the necessary thermal mass, it is important to design the envelope of the building to minimize heat loss and heat gain. On the heat loss side this involves insulating the walls, roof, and floors. Walls should be at least R-19, roofs should be at least R-30, and exposed floors should be at least R-11. It also involves minimizing infiltration of cold outside air and the accompanying exfiltration of warmed interior air. This is achieved by sealing up all the construction cracks around windows, doors, the top and bottom plates of stud walls, and openings through the ceiling into the attic. On the cooling side solar gain needs to be minimized through window orientation and appropriate shading devices.
Passive solar heating and cooling techniques apply best to buildings of modest size where the heating and cooling requirements are determined primarily by heat flows out of and into the building through the envelope of the building. Single family houses are the best example of this type of building, but other types like multifamily housing and small commercial buildings will approach an envelope dominated thermal flow building type.
FIGURE 14.1 Solar radiation by wall orientation and season.
Source: Moore 1993.
Since passive solar buildings will need to respond to their local macro and micro climates, it is not likely that what is good design in one location will be good design in another. The result is that energy efficient passive solar buildings can provide national, regional, and local diversity of form and design.
Direct gain is the simplest passive solar strategy (Figure 14.2). The building is oriented to the south. Remember that bioclimatic design suggests an orientation slightly east of south. The window area necessary is approximately 10 to 30 percent of the served floor area. The served floor area is the room that the solar gain is going into. Solar heat will not magically travel across a hall and into a room on the north side of a house. There needs to be an accompanying amount of thermal mass to soak up the solar gain or the room will overheat during the day and be cold at night. The appropriate amount of thermal mass is four to five times the window area, four inches thick. Four inches is the thickness of concrete that heat will flow into and out of in a 24 hour cycle of charging and discharging.
A computer energy simulation can be used to study how much thermal mass is necessary in a direct gain solar heating design (Figure 14.3). An Energy-10 simulation of a passive solar design for Sterling, Virginia, included an infinite insulation layer below the thermal mass concrete floor slab. For both the heating only and the heating and cooling simulations the energy benefit of thermal mass is not improved much beyond four inches thick. The optimum south glass area can also be explored with multiple computer simulation runs. For heating only the optimum window area is indicated by the flat bottom of the curve, which extends from about 16 to 24 percent of the floor area. For heating and cooling the optimum south glass area is in the range of 10 to 15 percent of the floor area (Figure 14.4).
FIGURE 14.2 Passive solar direct gain distributed mass.
The Passive Solar Energy Book (Mazria, 1979) provides the following window area ratios. For winters with average temperatures of 20 to 30 degrees Fahrenheit, set the south window area to 19 to 38 percent of the floor area. And for winters with average temperatures of 39 to 45 degrees Fahrenheit, set the south glass area to 11 to 25 percent of the floor area (Mazria 1979, 119).
The Solar Savings Fraction method suggests 17 to 35 percent south glass for Chicago, with 5,753 heating degree days and a heating design temperature of 3.1 degrees Fahrenheit. It suggests 12 to 23 percent south glass for Washington, DC, with 4,047 heating degree days and a heating design temperature of 20.2 degrees Fahrenheit. And finally, for the milder climate of San Francisco, it suggests 6 to 13 percent south glass with 3,016 heating degree days and a heating design temperature of 40 degrees Fahrenheit (Grondzik et al. 2010, 1646–1649).
The California Energy Commission Passive Solar Handbook, based on multiple Calpas computer simulations, suggests a range of 0 to 33 percent of the floor area in south glass (Niles and Haggard 1980, 90). The insulation level of a house has a large effect on heating energy use and on the amount of south glass area required for passive solar heating. Data from the California Energy Commission Passive Solar Handbook for Alturas, a cold climate, and for San Rafael, a mild climate, illustrates the importance of insulating a house very well before applying passive solar design methods (Figures 14.5 and 14.6).
FIGURE 14.3 Energy-10 simulations to determine the optimum concrete floor thickness when used as thermal mass. An R-1000 insulation layer was placed under the concrete floor, which was increased in thickness in inch increments.
FIGURE 14.4 Energy-10 simulations to determine optimum south window area for Sterling, Virginia. The floor area of the house was 2,000 square feet with 2,000 square feet of 4 inch thick concrete floor as thermal mass.
FIGURE 14.5 Computer simulations of passive solar glass area for Alturas, California, with 6,553 heating degree days for three different house insulation levels. The floor area of the house was 2,000 square feet with 2,000 square feet of 4 inch thick concrete floor as thermal mass.
Source: Data from Niles and Haggard 1980.