Energyfromthesun.ppt

Earth’s Surface Earth Orbit Solar System Mars Do you have any questions or topics you would like to discuss? For those interested in talking more, contact me at: joseph.c.kolecki@grc.nasa.gov Energy from the SunPart I Our sun is a star, one of a population of around 300-400 billion or so that make up the Milky Way Galaxy. It is not necessarily a distinguished star, as stars go, except for having the only planet know to humans as bearing life. Our sun is described, in terms of modern physics, as a sustained thermonuclear reaction that has been running for somewhere in the vicinity of 5 billion years, and will likely continue to do so for another 5 billion, give or take… With the advent of 20th century physics, we realized that nature can accomplish feats of energy conversion not dreamed of in earlier centuries. Specifically, solid state quantum theory led to the development of devices that can convert incident sunlight directly into usable electrical energy. In this series, we will touch on the basics of such energy conversion. [When sunlight strikes a surface, we say that it is incident upon that surface.] 1. The Sun’s Energy Output Before diving into the science of energy conversion, let us examine our sun and learn how its enormous energy output is figured in numerical terms by scientists and engineers. We shall begin with a number called the “Solar Constant.” The solar constant measures energy flux, that is, the amount of solar energy falling on a surface of a given area in a given time. The unit of energy chosen is the millijoule; the unit of area, the square centimeter; and the unit of time, the second. We can use a number of different devices to measure the solar constant, including black surfaces that collect solar energy and warm up. The result is that, every second on earth, approximately 1367 millijoules fall on a square centimeter oriented normal to solar incidence. [Let’s pause here to look at the terms: Sunlight has a direction – it flows outward fr