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 The Solar Car Book A complete kit for making a cool solar racecar. Everything is included: wheels, axles, motors, wires and a genuine one-volt solar cell. |
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Insolation & Solar Radiation Experiments
Insolation is a measure of solar radiation energy received on a given surface area in a given time. It is commonly expressed as average irradiance in watts per square meter (W/m²) or kilowatt-hours per square meter per day (kW·h/(m²·day)), or in the case of photovoltaics it is commonly measured as kWh/kWp•y (kilowatt hours per year per kilowatt peak rating). The surface may be a planet or a terrestrial object inside the atmosphere, or any object exposed to solar rays including spacecraft. Some of the solar radiation will be absorbed, causing radiant heating of the object, and the remainder will be reflected. The proportion of radiation reflected or absorbed depends on the object's reflectivity or albedo. Sometimes, as in the text below, a long-term average intensity of incoming solar radiation will be given in units such as watts per square meter (W/m2 or W·m-2) and called insolation, with the duration (such as daily, annual, or historical) stated or only implied.
Projection effectThe insolation into a surface is largest when the surface directly faces the Sun. As the angle increases between the direction normal to the surface and the direction of the rays of sunlight, the insolation is reduced in proportion to the cosine of the angle. This is known in optics as Lambert's cosine law. This 'projection effect' is the main reason why the polar regions are much colder than equatorial regions on Earth. On an annual average the poles receive less insolation than does the equator, because at the poles the Earth's surface is angled away from the Sun. Earth's insolationDirect insolation is the solar radiation that is transmitted directly through the atmosphere to the earth's surface without interacting with atmospheric components. Diffuse insolation is the solar radiation that is scattered or reflected by atmospheric components. Over the course of a year the average solar radiation arriving at the top of the Earth's atmosphere is roughly 1366 [1] watts per square meter (see solar constant). The radiant power is distributed across the entire electromagnetic spectrum, although most of the power is in the visible light portion of the spectrum. The Sun's rays are attenuated as they pass though the atmosphere, thus reducing the insolation at the Earth's surface to approximately 1000 watts per square meter for a surface perpendicular to the Sun's rays at sea level on a clear day. The actual figure varies with the Sun angle at different times of year, according to the distance the sunlight travels through the air, and depending on the extent of atmospheric haze and cloud cover. Ignoring clouds, the average insolation for the Earth is approximately 250 watts per square meter (6 (kW·h/m²)/day), taking into account the lower radiation intensity in early morning and evening, and its near-absence at night. The insolation of the sun can also be expressed in Suns, where one Sun equals 1000 W/m² at the point of arrival. One Sun is a unit of power flux, not a standard value for actual insolation. Sometimes this unit is referred to as a Sol, not to be confused with a sol, meaning one solar day on, for example, a different planet, such as Mars. ApplicationsIn spacecraft design and planetology, it is the primary variable affecting equilibrium temperature and global climate. In construction, insolation is an important consideration when designing a building for a particular climate. It is one of the most important climate variables for human comfort and building energy efficiency.[2] The projection effect can be used in architecture to design buildings that are cool in summer and warm in winter, by providing large vertical windows on the equator-facing side of the building (the south face, in the northern hemisphere): this maximizes insolation in the winter months when the Sun is low in the sky, and minimizes it in the summer when the noonday Sun is high in the sky. (The Sun's north/south path through the sky spans 47 degrees through the year). Insolation figures are used as an input to worksheets to size solar power systems for the location where they will be installed.[3] The figures can be obtained from an insolation map or by city or region from insolation tables that were generated with historical data over the last 30-50 years. Photovoltaic panels are rated under standard conditions to determine the Wp rating (watts peak),[4] which can then be used with the insolation of a region to determine the expected output, along with other factors such as tilt, tracking and shading (which can be included to create the installed Wp rating).[5] Insolation values range from 800 to 950 in Norway to 2200-2400 kWh/kWp•y in Thailand and Israel. In the fields of civil engineering and hydrology, numerical models of snowmelt runoff use observations of insolation. This permits estimation of the rate at water is released from a melting snowpack. Field measurement is accomplished using a pyranometer. See alsoReferences
External links Look up insolation in Wiktionary, the free dictionary.
Solar radiation is radiant energy emitted by the sun from a nuclear fusion reaction that creates electromagnetic energy. The spectrum of solar radiation is close to that of a black body with a temperature of about 5800 K. About half of the radiation is in the visible short-wave part of the electromagnetic spectrum. The other half is mostly in the near-infrared part, with some in the ultraviolet part of the spectrum. [1] When ultraviolet radiation is not absorbed by the atmosphere or other protective coating, it can cause a change in the skin color of humans. Solar radiation is commonly measured with a pyranometer or pyrheliometer.
Solar constant
The solar constant is the amount of incoming solar electromagnetic radiation per unit area, measured on the outer surface of Earth's atmosphere, in a plane perpendicular to the rays. The solar constant includes all types of solar radiation, not just the visible light. It is measured by satellite to be roughly 1366 watts per square meter,[2] though it fluctuates by about 6.9% during a year - from 1412 W/m2 in early January to 1321 W/m2 in early July, due to the earth's varying distance from the sun, and by a few parts per thousand from day to day. Thus, for the whole Earth, with a cross section of 127,400,000 km², the power is 1.740×1017 W, plus or minus 3.5%. The solar constant is not quite constant over long time periods either; see solar variation. The value 1366 W/m2 is equivalent to 1.96 calories per minute per square centimeter, which can also be expressed as 1.96 langleys (or Ly) per minute. The Earth receives a total amount of radiation determined by its cross section (π R2), but as the planet rotates this energy is distributed across the entire surface area (4 π R2). Hence, the average incoming solar radiation (called sometimes the solar irradiance), taking into account the half of the planet not receiving any solar radiation at all, is one fourth the solar constant or ~342 W/m². At any given location and time, the amount received at the surface depends on the state of the atmosphere and the latitude. The solar constant includes all wavelengths of solar electromagnetic radiation, not just the visible light. (See electromagnetic spectrum for more details) It is linked to the apparent magnitude of the Sun, −26.8, in that the solar constant and the magnitude of the sun are two methods of describing the apparent brightness of the Sun, though the magnitude only measures the visual output of the Sun. In 1884 Samuel Pierpont Langley attempted to estimate the solar constant from Mount Whitney in California, and (by taking readings at different times of day) attempted to remove atmospheric absorption effects. However he obtained the incorrect value of 2903 W/m2, perhaps due to mathematical errors. Between 1902 and 1957, measurements by Charles Greeley Abbot and others at various high-altitude sites found values between 1322 and 1465 W/m2. Abbott proved that one of Langley's corrections was erroneously applied, and his results varied between 1.89 and 2.22 calories (1318 to 1548 W/m2), and the variation appeared to be solar, not terrestrial.[3] The angular diameter of Earth seen from the sun is ca. 1/11,000 radian, so the solid angle of Earth seen from the sun is ca. 1/140,000,000 steradian. Thus, the sun emits about 2 billion times the amount of radiation that is caught by Earth, or about 3.86×1026 watts.[4] Climate effect of solar radiation
On Earth, solar radiation is obvious as daylight when the sun is above the horizon. This is during daytime, and also in summer near the poles at night, but not at all in winter near the poles. When the direct radiation is not blocked by clouds, it is experienced as sunshine, a combination of bright yellow light (sunlight in the strict sense) and heat. The heat on the body, on objects, etc., that is directly produced by the radiation should be distinguished from the increase in air temperature. The amount of radiation intercepted by a planetary body varies inversely with the square of the distance between the star and the planet. The Earth's orbit and obliquity change with time (over thousands of years), sometimes forming a nearly perfect circle, and at other times stretching out to an orbital eccentricity of 5% (currently 1.67%). The total insolation remains almost constant but the seasonal and latitudinal distribution and intensity of solar radiation received at the Earth's surface also varies [5]. For example, at latitudes of 65 degrees the change in solar energy in summer & winter can vary by more than 25% as a result of the Earth's orbital variation. Because changes in winter and summer tend to offset, the change in the annual average insolation at any given location is near zero, but the redistribution of energy between summer and winter does strongly affect the intensity of seasonal cycles. Such changes associated with the redistribution of solar energy are considered a likely cause for the coming and going of recent ice ages (see: Milankovitch cycles). Notes
See also
External links
This article is licensed under the GNU Free Documentation License. It uses material from Wikipedia Encyclopedia article "Insolation" |
