Thermodynamics Question?

2009-07-10 9:49 am

1)Determine the work available in a 3.1 -kg block of copper at 490 K if the surroundings are at 300 K. (relates to a theorem that states how much energy is unavailable to do work)

2) An ideal air conditioner keeps the temperature inside a room at 23 C when the outside temperature is 39 C. If 3.0 kW of power enters a room through the windows in the form of direct radiation from the Sun, how much electrical power would be saved if the windows were shaded so only 900 W came through them?

回答 (1)

[匿名]

2009-07-10 10:24 am

✔ 最佳答案

1)
Copper Cp = 0.385 kJ/kg K
For thermal equilibrium with sorroundings
Heat available = m Cp * dT = 3.1 * 0.385* (490-300) = 226.765 kJ

Work available = (1- Tc/Th) * heat available = (1-300/490)*226.765
= 87.93 kJ
-----------------------------

2) COP =Tc/(Th-Tc) = (273+23)/ (273+39 - 273 +23)
= 296/ 62 = 4.77

Also COP= Heat removed / Power input
so initial power P1 = H/4.77
final power with reduced heat load P2 = [H-(3-0.9)]/4.77

P2-P1 =( H- 2.1 - H)/4.77
Power reduction = P2-P1 = - 2.1 / 4.77 = - 0.44 kW

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