PV Gap

Misty Said:

How is the study of radiative heat transfer associated with solar power generation?

We Answered:

More accurately, the sun radiates energy as electromagnetic waves, mainly at the wavelengths of visible light, which can be absorbed by a suitable panel or collector, and converted to other energy forms such as heat or electricity. There is not much heat radiation (infra red) because the sun is too hot to radiate a lot of infra red. This is associated with "black body radiation" (see the link). A black body at room temperature of 300K emits ~10,000nm wavelength radiation, whereas the sun at ~6000K is ~560nm radiation.

When we feel the sun warming our skin that is what is happening. The direct visible light from the direction of the sun represents about 1000 watts per square meter at the bottom of the atmosphere. It gets absorbed and reflected more or less by everything the light impinges on. Sometimes the resultant heating causes infra-red (heat) energy to be re-radiated, so when we put our hand near a building wall or other object in the sun, the wall feels hot.

With photovoltaic panels, part of the light from the sun is converted to electricity, and part heats the panel. The panel needs suitable cooling by convection and radiation to ensure the heating is not excessive.

With solar absorption panels (for hot water or more seriously hot for generating electricity using a heat engine), the light from the sun is collected and absorbed and so used to heat the working fluid. The working fluid can be further heated by recirculation of the fluid or by focusing the sunlight. With high temperatures involved >500C in some cases) the power per unit area is increased by focusing the sun, so the power density is greater. If a 10 meter diameter parabolic reflector is used to focus the sun onto a 200mm x 200mm absorption block, the area ratio is:
For the dish, pi x r^2 = 78.5m^2
For the absorption block length x breadth = 0.04m^2
Ratio 78.5/0.04 = 1962.5
The ratio is almost 2000:1, so the energy density is now ~2MW per square meter (less any losses). This can heat a working fluid to a high temperature rapidly as the heat flows from the absorption block into the working fluid much faster. With heat, the flow is in joules per second = watt-seconds/second = watts.

Nathan Said:

Whether solar power generation is feasible in warm countries?

We Answered:

Photovoltaic panels do become somewhat less efficient at high temperatures, but that is more than offset by increased sun hours near the equator. I think the chief concern would be cloud cover more than heat. The windward parts of Oahu (Hawaii) don't get any more sun than Northern California, because of increased cloud cover and rain.

Clifton Said:

I need answer from the experts regarding the solar power generation in India, regarding development and issues

We Answered:

contact the barefoot college

Jacob Said:

How to get information about lower scale wind and solar power generation forms?

We Answered:

There is plenty of information out on the internet about renewable energy generation for residential applications.

Here is a great source of information on wind turbines for residential use: http://www.residential-wind-power.com

There is also a great article here on Do It Yourself Wind Turbines:
http://www.residential-wind-power.com/2008/06/top-5-do-it-yourself-wind-turbines/

Margie Said:

How long it would take for solar power generation to pay for itself?

We Answered:

This answer is very sensitive to location for two reasons:

1) Amount of sunlight, and
2) Prevailing price of electricity.

Payback is based on the local value of the electricity generated.

On an island in the Pacific where all electricity comes from diesel fuel imported by private boat, electricity may be $1 per kWh. Solar could pay for itself in just 2 years, there. Such places actually exist.

Where electricity is very cheap, or sunlight is scare (think of long northern winter nights, or places that rain all the time), solar may never pay for itself over the life of the system.