Specific Heat


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HEAT is the product of temperature and mass. It is the sum total of molecular motion. Specific heat measures the amount of heat a substance can store. Temperature measures the average kinetic state of molecules. Fast moving molecules are hot and slow moving molecules are cold. If we multiply the average kinetic state of molecules by its mass we are measuring heat. Q = Cp X (T1-T2)

Some people feel that heating stones with solar heated air is the best method of exchanging and storing heat, but stones, glass, marble and cement have a low specific heat of about .2 cal/g �C. This method of heat storage may be practical in desert areas where sunlight is plentiful and space is not a major concern, but in cold climates where prolonged heat storage is required water is a more practical storage medium since the heat capacity (Cp) of water is so high. 

Substance

Specific Heat in:   cal/g/C* or  BTU/lbs./F*

Air

0.24

Asphalt

0.22

Bone

0.11

Water, solid

0.50

Granite

0.19

Sandy clay

0.33

Quartz sand

0.19

Water, liquid

1.00

Wet mud

0.60

Wood

0.41

 

CONDUCTION: Solid objects transfer their vibrating molecular motion by conduction. Some substances conduct heat better than others. Aluminum, Copper and Silver are excellent conductors of heat. Collectors convert radiant energy into heat and then transfer that heat to absorber plates, flow tubes and collector fluid by the process of conduction. 

CONVECTION: Liquids and gases transfer heat by convection currents. Hot molecules rise and cold molecules fall. This is how a passive solar greenhouse is able to heat a house.

HEAT TRANSFER:  The ability to transfer and store heat depends on the mass of a substance and also something called the specific heat.  Specific Heat is defined as the amount of heat needed to raise 1 g of a substance 1� C.  or 1 pound of water 1 F. The specific heat of water is 1.00 cal/g �C and the specific heat of air is .24 cal/g �C. Since air is a thousand times less dense than water and the specific heat of water is four times higher than air the same volume of water and can store  4,000 times more heat than air.

13 million, million tons of water in the atmosphere are responsible for about 70% of all atmospheric absorption of radiation, mainly in the infrared region where water shows strong absorption. It's the water in the atmosphere that keeps our planet warm enough to support life.... but how?

Without water life as we know it would be impossible. There are many qualities of water that make it special but it's response to IR radiation at frequencies around 3300 cm-1 are the qualities that make storing heat energy practical. Since the stretching frequency of the -OH bonds have the capacity to store kinetic energy and water has two -OH bonds per molecule water has a very high heat capacity.

Phase Change Materials are sometimes used where space is limited for storing heat. PCMs have a high heat of fusion which means they give off a great deal of heat when they melt at a certain temperature. In other words heat is absorbed or released when the material changes from solid to liquid or from liquid to solid. Sodium Sulfate Decahydrate or Glauber salt is a commonly used for this purpose since it changes phase around 90 �F and makes this material especially appropriate for storing low grade solar heat for later release in space heating applications. In some applications the PCMs are incorporated into thermal tiles that are placed in an attic space while in other applications the salt is incorporated into cells surrounded by solar�heated water. The phase change allows a substantial reduction in the mass of the material required for effective heat storage.  HOWEVER the heat transfer process is more involved than using PCMs alone as a heat storage mediums is not always practical.

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