Book VII: Solar Power in VS .NET

Generate Denso QR Bar Code in VS .NET Book VII: Solar Power

Conduction: In conduction, energy is transferred between molecules within a substance Only heat moves, not the molecules (Well, molecules are always moving, but the molecules tend to vibrate in place rather than switch places) Heat always moves from a hotter surface to a cooler surface (Technically speaking, heat moves in both directions, but statistically the most movement occurs from hotter to cooler) Copper is an excellent conductor of heat In fact, most good conductors of electricity are also good heat conductors Glass is a poor conductor of heat and electricity and is therefore considered an insulator Convection: Convection is the transfer of heat between a fixed, rigid surface and a moving fluid in contact with that surface or the transfer of heat as molecules move from one point to another within the fluid A fluid is any substance that can flow, so air is also considered a fluid As opposed to conduction, the hot molecules themselves move and switch places Water flowing over a hot surface becomes hot In a tank of water, the hotter molecules expand and weigh less and move upward while gravity draws the colder, heavier molecules down This heat exchange is called thermosiphon, and it s a type of convection Radiation: A hot object emits infrared radiation The sun is a perfect radiator Fires radiate heat, a lot of which you can see A really hot fire glows white; all wavelengths in the spectrum are present As the fire cools down, the color changes to orange, then to red, and finally, when you can t see the embers anymore but can still feel the heat, the radiation is entirely infrared The colors reflect the energy content of the fire

Solar heat collectors are designed to collect as much solar radiation as possible Not only do you want to maximize surface area, but you also want to orient the collector in a way that maximizes the amount of solar radiation gathered over the course of a day Solar heat collectors also ought to do the following, all of which affect what kind of materials go where in your solar collector: Convert that radiation into heat as efficiently as possible This conversion almost always entails the use of a black surface, which absorbs radiation efficiently Transfer the heat into a usable medium In most cases, you want to heat water If you collect the heat in a black tube, you can simply run water through that tube, and the heat naturally transfers into the water You can also use other fluids, such as antifreeze fluid To move heat effectively, a material needs to be a good conductor (or as physicists like to say, it needs to have high thermal conductivity)

Insulate to prevent heat loss For insulators, you want to use materials that make for poor conductors Converting radiation into heat in the wintertime is easy, but then the cold weather and wind simply steal the heat right back In order to prevent this loss, people insulate the collector from the outside world with materials, such as fiberglass insulation, or double-pane glass windows Store a sufficient amount of heat Heat capacity is a measure of how much heat energy (in British thermal units, or Btus) is required to raise the temperature of a given volume of a material by 1 F To be effective at storing heat, a material needs a high heat capacity Table 2-1 shows some materials of interest

Table 2-1

Heat Capacity (Btus per Cubic Foot F) 0018 057 22 22 25 268 59 624 78

So the ideal material for storing and moving heat needs to have a high heat capacity and also has to be a good conductor Although wood and concrete store a lot of heat, you can t move it into or out of these materials very easily Steel is very good, but it rusts Of course, water is the cheapest, so it s used to store and transfer heat, but the problem is that it doesn t absorb radiation because it s clear Copper painted black absorbs, stores, and conducts heat extremely well, so copper is used extensively in solar heat collectors