Heat Transfer (Solar Radiation)

In summary, the student is trying to solve for the solar radiation coming into a room, given the temperature of the sun, the temperature of the room, and the surface emissivity. They are not sure how to calculate the alpha of the surface.
  • #1
Chibus
5
0

Homework Statement



This questions relates to solar radiation and heat balance. I'm having trouble with part (b) of the question, having solved (a) already.

The question is found in the following image:

xogvvb.jpg


Homework Equations





The Attempt at a Solution



Qconv=h*A*(T-Tambient)=420 W

Qconv=420 W
A=2m^2
Tambient=303 K

Given T=(The equation found in the link)^(1/4)

Emissivity=0.9
Ne=0.15
Gsn=1000 W/m^2
Aplate=2m^2
Stefan-Boltzman Constant=5.7E-8

The only thing I need is the abosrbance (alpha) of the surface.

I'm really unsure of how to calculate it, given I don't have the temperature of the sun.

Can anyone shed some light?



On a related note, regarding Kirchoffs law of thermal radiation:

Emissivity=Absorbance, given equilibrium temperature.

I'm having some trouble understanding where I can use this relationship. For example, in the given problem, I know that in the instant of time being looked at, the system is in equilibrium. Thus, I can say that the emissivity of the surface is equal to the absorbivity, however, this relationship doesn't apply when I am considering radiation coming in from the sun?

The following is taken from a section of my lecture notes describing the radiation function and the solar absorbance:

i.e. I evalulate the absorbance of the surface at the temperature of the sun, but I evaluate the emissivity at the equilibrium temperature of the surface. I don't understand why this is so -

rlw8dd.png


Sorry fellas, just getting pretty confused here. Any help would be eternally appreciated.
 
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  • #2
If I remember my thermodynamics properly, for a grey body [itex]\alpha = \epsilon[/itex]. Which applies to any radiating surface I believe.
 
  • #3
The convective heat flux (q)=h*A*(Ts-Tinf) where Ts is the surface temp(a above), Tinf is the ambient air temp. h is the convection heat transfer coefficient. A is the area. You know q, A and both T's, looks like plug-and- chug to me.
 

1. How does solar radiation affect Earth's climate?

Solar radiation is a major driving force behind Earth's climate as it is responsible for warming the planet and driving global weather patterns. The sun's energy is absorbed by the Earth's land, water, and atmosphere, which then re-radiates heat back into the atmosphere. This process creates a balance of heat on Earth and determines the planet's overall temperature and climate.

2. What is the difference between conduction, convection, and radiation in terms of heat transfer?

Conduction is the transfer of heat through direct contact between two objects, such as a hot pan heating up the food in it. Convection is the transfer of heat through the movement of fluids or gases, such as warm air rising and cool air sinking. Radiation is the transfer of heat through electromagnetic waves, such as the heat from the sun reaching the Earth.

3. How does the Earth's atmosphere affect the amount of solar radiation that reaches the surface?

The Earth's atmosphere plays an important role in regulating the amount of solar radiation that reaches the surface. Some of the sun's energy is reflected back into space by clouds, gases, and particles in the atmosphere. The rest is absorbed by the atmosphere and the Earth's surface, which then re-radiates heat back into the atmosphere. Greenhouse gases, such as carbon dioxide and water vapor, trap some of this heat and contribute to the Earth's overall temperature.

4. How does the tilt of the Earth's axis affect the amount of solar radiation received by different parts of the planet?

The tilt of the Earth's axis is the reason for the changing seasons on our planet. As the Earth rotates around the sun, different parts of the planet are tilted towards or away from the sun at different times of the year. This tilt affects the angle at which the sun's rays hit the Earth's surface, resulting in variations in the amount of solar radiation received by different regions throughout the year.

5. How do humans impact the amount of solar radiation that reaches the Earth's surface?

Human activities, such as deforestation and the burning of fossil fuels, can contribute to an increase in the amount of carbon dioxide and other greenhouse gases in the atmosphere. This can lead to an enhanced greenhouse effect, trapping more heat in the atmosphere and causing an increase in the Earth's overall temperature. This can also result in changes in weather patterns and an increase in global temperatures, known as global warming.

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