PV Gap

Michael Said:

HOw i measure a energy eficiency of a solar box oven ? How we measure de energy eficiency of a stove?

We Answered:

Its simple in theory, harder to actually do. First off the energy in a system is not lost or destroyed

Here is the break down of what needs to be done.
1. Find or calculate the energy going into the oven.

2. Find mass of air within the oven.
Find the volume (in meters) of the inside of the oven. Now take the volume and multiply it by the density of air (at sea-level it is 1.2 kg / m^3) to give us the mass

3. Find the tempurature change in the oven over a minute

4. Now calculate the energy it takes to heat the air
Heat gained (energy) = mass of being heated (step 2) X change in Temp (step 3) X specific heat index (air is 0.25)
1 Joule = 0.239 cal

5. Calc the Work done
Work = energy (step 4 in Joules) / time (step 3 is 60 sec)

6. Calc the Energy Efficiency
E = Work done (step 5) / Work used (step 1 divided by 60 sec)

Cassandra Said:

In calculations of the sun's heating of the earth,1367 watts/m2 is used for the solar constant. What time unit?

We Answered:

The important thing is the energy. So it is joules/ (sec *m^2). If you have this you can calculate the total energy input from the sun for a given time period for the entire globe. Then once you subtract the energy lost, you can calculate the temperature gain or decline. Naturally it is much more complex than that but that is the general idea.

Kilowatt- Hours is a unit of energy. You charged for how much energy you consume. It is perhaps a screwy unit but it is the standard used. In my engineering work, we sometimes have units that mix metric and english units. This doesn't make much sense but it is the standard that has been used and nobody wants to change it.

Sharon Said:

follow up on solar energy size question to replace imports?

We Answered:

There was a very good article in Scientific America perhaps 6 months ago. The authors calculated (from my possibly faulty memory) that for $10 Billion, in 20 years, and assuming lowering costs for solar arrays, we could build enough solar arrays in the SW of the country, where you get a lot of sun, and land costs are low, to replace a substantial portion of the power needs plus enough for normal expansion. This would include new power transmission systems to get the power to the east.

Don't forget our grid is very marginal now, and we need a lot of new capacity in the next 10-20 years just to keep up with increasing demand.

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Hazel Said:

What co-efficient of the sun's energy is absorbed by the Earth's atmosphere?

We Answered:

As Mis37N noted; about 30% of the solar radiation is reflected back out into space. Of the remaining radiation, about 2/3 (ie, about 45% of the total) makes it to the ground. That means that the atmosphere absorbs about 25% of the solar radiation; or about 33% of the radiation that is not reflected back into space.

The absorption is quite specific: the higher levels of the atmosphere absorb all the gamma ray and X ray; and most of the ultraviolet. The lower atmosphere absorbs all of the infrared and microwave radiation. There is a narrow band of visible light that passes straight through; plus the longer wavelength radio waves.

Note that the absorption of the infrared by the lower atmosphere is what causes the greenhouse effect: the visible light passes straight through, but the radiated heat (infrared) is absorbed. This is a good thing, because without the greenhouse effect; life as we know it wouldn't exist. But you can have too much of a good thing.
If you want further information, have a look at this excellent site:
http://www.windows.ucar.edu/tour/link=/e…

Joy Said:

solar energy buildings?

We Answered:

The whole thing is based on need, solar cells produce current at a low voltage and current density, just for argument 2 volts and 100 milliamps each, it would take a vast array to provide space heating for a house on a mountain in Tibet, but a small array charging a bank of 3 truck batteries could easily provide a laptop computer and six LED type lamps for a simple home in a warm climate. SO the selection of the source of power is dependent on the end use. BIO GAS is one answer for some things, Solar water heating is an answer in other cases, it has to be tayloredto the end use. READ the book by RB Fuller to learn about his notions from a long time ago about this sort of thing. He may not have all the answers but he was right there with most of his designs, just 50 years too soon...Maybe you can walk in his footsteps! SENERGY!