passives solar design
Passive Solar Design in Indian Cliff Dwellings

Figure 1: American Indian culture that roamed the Southwest from 1200 B.C. to A.D. 1300 built cliff dwellings such as these at Manitou Springs Colorado. With a southwest exposure and overhanging cliffs they are excellent examples of passive solar design.

the basics

The ancient Native American cliff dwellers understood more about passive solar design than most modern architects and builders. Before the days of oil-burning furnaces, cheap electricity, and triple-glazed windows, Native Americans had to rely on passive solar energy for both heating and cooling of their dwellings. In the Southwest, they built into cliffs. Most of these cliff dwellings were erected on south-facing ledges in deep sandstone canyons. Thanks to their southern exposure, the low-riding sun provided heat during the winter and the overhanging cliff above offered cool shade from the high summer sun.

This guide describes to builders, architects and homeowners techniques they can employ to dramatically reduce the energy footprint of structures that they build. The techniques do not rely on advanced and expensive technologies such as photovoltaicpanels, triple glazed solar windows, or a foot or more of insulation. Many of the gains, are achieved by applying simple design practices and by employing materials like concrete and water. There is an infinite number of combinations of these simple solutions, with the best often determined by the site location, the local year-round climate, and the construction budget. But before we discuss specific design practices it is beneficial to understand solar energy.

solar energy

The sun's energy arrives on earth in the form of heat and light. This solar radiation travels through space in waves, and it is the wavelength of these light waves that what we call the solar spectrum. There are two important facets you need to consider about the solar spectrum. First, while the sun emits radiation in all wavelengths (from less than a millionth of an inch to more than a thousand yards), it is the shorter wavelengths that account for the majority of energy in the solar spectrum. For example, the portion of the spectrum we see as visible light is a relatively small segment, yet accounts for 46 percent of the energy radiating from the sun. Another 49 percent, that which we perceive as heat, is derived from the relatively narrow infrared band of the spectrum. Because the proportion of different wavelengths in the solar spectrum is fixed, the energy output of the sun remains constant. This phenomena is known as the Solar Constant. We define this as the amount of heat energy delivered in one hour to a square foot perpendicular to the sun's rays at the outer edge of the earth's atmosphere. The Solar Constant measurement is roughly 430 BTUs/ft2 with minimal changes over the year. However, the Solar Constant is not a measure of the amount of solar energy that actually reaches the earth's surface. Approximately 35% of all the solar radiation intercepted by the earth's atmosphere is reflected back into space. Additionally, water vapor and atmospheric gases absorb another 15%. As a global average, only 35-40% (150 BTUs/ft2) of solar radiation entering the atmosphere actually reaches the earthÕs surface. This average, however, varies based on the season and the time of day, Both variables determine how much atmosphere the solar radiation must travel through before striking any particular location on the globe. Seasonality sets the upper limit amount of solar energy that can reach the surface of the earth at any location on any day of the year.

energy density

As mentioned, one of the conditions for accurately measuring the Solar Constant requires that the intercepting surface be perpendicular to the sun's rays. Any deviation from the perpendicular reduces the radiation density and the corresponding amount of energy intercepted. This is best illustrated in Figure 2. The angle created by incoming radiation and a line perpendicular to an intercepting surface is called the angle of incidence. The perpendicular distance (D) is less than the angular distance (D+) meaning more surface energy hits near the equator than near the poles. Because of the earth's tilt on it's axis, it also means more radiation falls on a location in the summer than in the winter since distance (D+) is shorter. Luckily for people living in higher latitudes, a fairly large increase in the angle of incidence results in only a modest reduction in the energy density.

Solar Constant at edge of earth's atmosphere

Figure 2: Solar constant at edge of earth's atmosphere is greater than radiation that hits surface of the earth due to attenuation.

radiation & surfaces

When sunlight strikes a surface it is either reflected, transmitted or absorbed, in any combination depending on the texture, color and clarity of the surface. All completely opaque surfaces both reflect and absorb radiation but do so in different ways. For example, a rough surface such as brick reflects scattered sunlight while a smooth, glossy surface reflects uniformly and at a reflection angle equal to the angle of incidence (think of a mirror). The wavelengths of solar radiation that are reflected are determined by the color of the surface material. A red brick surface, for example, will scatter (diffuse) wavelengths in the red band of the spectrum and absorb all others, while a white glossy surface will reflect all wavelengths in the visible spectrum at an angle equal and opposite to the angle of incidence. Conversely, a rough black surface absorbs all wavelengths in the visible spectrum. The transparent surface of window glass allows nearly all radiation to pass through it with comparatively little reflection or adsorption, and without deflecting it from its parallel lines of travel. Translucent materials also transmit radiation but scatter the rays as they pass. It should be noted that few materials are either excellent reflectors, transmitters, or absorbers of the sun's rays.

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