passives solar design

solar absorption

Sunlight exhibits a transformation from solar energy to heat energy when it falls on any material. As a material absorbs solar energy it stimulates movement of the molecules in the material generating heat. The greater the movement, the greater the heat. We call this process absorption. In physics, absorption refers to the full spectrum. With passive solar, it refers to absorption over a specific spectral range of sunlight here on earth. Since the color black absorbs more of the light spectrum than the color white (thus more energy), black surfaces will be hotter than white surfaces. A flat black paint can have an absorption factor as high as 98%. This means that 98% of the incident solar energy is converted into heat. The radiation that is not absorbed, is lost due to reflection. Mirrors or polished metals are poor absorbers (absorption of 5 to 10%), since most of the light is reflected rather than absorbed. Table 1 shows absorption factors of some common, solar absorbers and finishes such as paint.

heat conduction

As a material absorbs radiation, it will redistribute heat energy due to the natural phenomenon of maintaining equilibrium. This occurs when stimulated molecules vibrating at a faster rate impact adjacent molecules vibrating at a slower rate. In this way, heat is conducted away from the source of energy and the energy distributes evenly throughout the mass. The rate at which energy "flows" or is conducted though a material depends on the density of the material and its conduction rate. Gases are poor conductors; metals are comparatively good conductors; and less dense materials containing tiny air pockets and voids (insulators) conduct heat at the least rate. In passive solar designs, you want the "C student" when it comes to conductivity. You need the heat storage material to absorb the heat over the course of the day, and then slowly release it over several hours at night. Concrete, bricks, and wood all satisfy this requirement.

heat transfer

Heat transfer from a solid material to a fluid medium (liquid or gas) occurs by radiation (infrared). Fluids can also move across a hot solid surface, allowing molecules of the fluid to become agitated (heated), then move away from the heat source, and be replaced by new, unheated molecules. When no machinery aids the process it is called natural convection. With an aid such as a fan it is called forced convection. Boiling water is a good example natural convection with heated molecules near the burner rising quickly to the surface where they boill off. Steam generated by boiling is simply water molecules whose vibration rate is violent enough to allow them to breakaway from of the water's surface. Heat rising off pavement is also evidence of the power of natural convection. Air heated by dark pavement rises, creating a shimmering or "mirage" effect when viewed from afar.

adsorption and emisstivity of common materials

Figure 3: Absorption (yellow) and emissitivity (red) of common building materials and solar thermal panels. A high absorption and low emissitivity means the material captures and keeps most of the incident solar radiation.


Molecular movement is continually generating heat in the form of radiant energy. Unlike solar energy, radiant energy is limited to low-temperature infrared radiation. The emission of thermal energy depends both on the temperature of the material and its surface. Polished metal surfaces have low thermal emission but also poor absorption of solar radiation. In contrast, black metal surfaces are the best absorbers but sometimes have high emission. Flat black paint, for example, while cheap to apply, it not necessarily the best material to use for a solar collector. It absorbs well, but also emits just as well.

solar characteristics of commmon building materials

In selecting passive solar materials you need to consider both absorption and emissivity. The higher the absorption the more you collect; the lower the emissivity, the less you lose. Ideally the emissivity should be zero. Of course that would be impossible. The best alternative is to look for materials that have high absorption and low emissivity. Brick, for example, will absorb 65% of incident radiation and release 88% of that right back at the surface (Figure 3). That leaves only 12% to be stored for later use. The "ideal" surfaces for absorbing and not emitting solar radiation are specialized coated metals (copper and aluminum) used in Zonbak solar thermal panels. The absorption is close to 100% and the emissivity is as low as 10%, meaning the material will convert almost all the solar radiation to heat and lose only 10% of it back from its surface. However, since they have a such small thermal mass, they will not store the heat. It must be transferred to a heat storage material (concrete, water, etc.) via passive or active convection.

Glass has the special characteristic of transmitting nearly all solar radiation. Solar energy passes through windows and is reradiated into an interior space in the form of thermal energy (heat). The heat is unable to pass back through the glass to the outside since glass blocks infrared thermal radiation. This is known as the greenhouse effect.

heat storage

All materials can store heat which is called its specific heat; the amount of heat, measured in BTUs for a given mass, a material can hold when its temperature is raised by one degree Fahrenheit. However, the specific heat of a material is not very useful when evaluating materials for passive solar designs. The more useful measure of a solar material is its heat capacity; a measurement of the specific heat of a material multiplied by its density. The higher the heat capacity, the more effective the material is for solar thermal storage. A good thermal storage material must absorb heat when it is available (daytime), and give it up when it is needed (nighttime). The rate at which a material absorbs heat is known as its conductivity.

specific heat capacity of commmon building materials

Table 2 shows the comparative specific heat measurements for a variety of common building materials. Unfortunately, there is no perfect storage medium in terms of volume, storage capacity, conductivity, and cost. Oak, for example is fairly good at storing heat, but it is too expensive to use alone as a heat storage material. Water has one of the highest heat capacities and is basically free, but it is much more difficult from a maintenance standpoint. You need containers to store it, it freezes, and under pressure the water storage system tends to leak and cause damage. Plus "stagnant" water will start to grow bacteria if left unattended.

Ideally, although not entirely a passive design, one approach is to collect heat with a selective solar material and use forced or natural convection to transfer that heat to an inexpensive, high heat capacity material such as concrete or water. Although not entirely "passive", the system may require nothing more than some air ducts and a fan. This capture, transport, and store approach uses materials with the highest absorption, conductivity and lowest emissivity, combined with the materials with high specific heat capacity such as concrete or water. Figure 4 shows a system where roof-mounted Zonbak solar thermal panels absorb solar radiation, convert it to heat, which is then transferred to the high heat-capacity concrete walls in the basement via forced-air ducts. The concrete will store the heat and then release it during the night. Floor vents can provide a path for passive convection currents to distribute the heat throughout the structure. Alternatively, the heat can be stored in an even higher heat-capacity material, water. An air-to-water heat exchanger, however, would be required to exchange the heat stored in the hot air (a gas) to water (a liquid). The water could then be stored in an insulated tank for both space heating and hot water needs.

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