PV Gap

Constance Said:

how do you measure energy efficiency?

We Answered:

efficiency= (Useful energy/Total energy) x100
that may not help but the question is hard to understand, for instance, what efficiency are you trying to find, the panel or the fan?

Phillip Said:

What is the percent energy conversion efficiency of these energy sources? Please...?

We Answered:

http://en.wikipedia.org/wiki/Image:PVeff…
shows "Sharp" cells at 15%, and I take this to be about what *commercially available* cells could do.
Another measure of efficiency is Energy Return On Energy Invested (EROEI). By this measure, a 5% cell could be good, if it is easy to make.

Solar-thermal technologies (similar to Kramer Junction) can be expected to do about 30%, with no breakthroughs or exotic materials.

Plants (wood) do about 2%, because they make about 5% converting sun to wood, then 30% of that to convert wood to heat to electricity. Cogeneration increases efficiency because it uses the waste heat from electrical generation. Recent rumors have that algae much more efficient than land plants.

Fusion cannot produce enough energy to keep itself going. EROEI is way less than 1.

Fission converts about 30% of a tiny fraction of the mass of special nuclear materials to electricity. The other 70% is heat.

Fossil fuels and are basicly plants that have been stored a long time (see above). Biofuels (biomass) have not been stored so long.

GeoThermal increases the efficiency of electricity by pulling heat from the ground or storing it there. It is not so much an energy source as a way of virtually putting a structure underground, where it is more insulated.

http://www.eroei.com/eroei/evaluations/n…
has an EROEI table.

Janice Said:

What is 100% efficiency in ref to solar power? How is it measured or determined?

We Answered:

A solar cell's efficiency is the percentage of incident light energy that is converted to electric energy. 31% is actually really good for a solar cell, most of them are under 20.
As for a theoretical 100% cell, the Second Law of Thermodynamics forbids any process from being perfectly efficient, but we'll keep getting closer and closer...

Carolyn Said:

Calculating Energy efficiency?

We Answered:

Effectiveness would be the difference in temperatures between identical pots one sitting in the sun by itself and one in the solar oven. Basically, what you want to measure is the thermal gain over doing nothing. (see 1st link for formulas)

Note that there is a difference between efficiency and effectiveness. An efficient oven may harvest more heat for a given size but not be large enough to cook with. The Vdara in Vegas may not be a very efficient solar cooker but its large parabolic shape facing south has made it an effective solar concentrator. (see 2nd link)

Energy loss would be the loss of captured heat from the oven.
However, some controlled loss may be desirable. You don’t want the oven to get too hot and burn your cookies.