Hi skyhawk,
Welcome to the forum!
Unfortunately, I have to ask you a few questions to be sure if what you're asking us -- but I will make a guess and try to provide some resources that might help you.
--I'm not sure what you mean by "triangler" -- is this a particular device, or are you interested in comparing a triangular magnifying lens with magnifying lenses of different shapes?
--When you say "more power", are you interested in the ability to concentrate to a small point, or in increasing the total energy absorbed by a solar panel if you place a magnifying lens above it?
Basically, what a lens can do (in the ideal case) is take all the light energy that strikes its surface and focus it down to a point some distance below it. But, a little bit above or below the plane of that focal point, the rays bent by the lens will be spread over some finite area. (Also, the angle by which light is bent depends slightly on its wavelength, so the focal plane will be at a different distance away from the lens for each wavelength. Probably a smaller effect that the difficulty of making a geometrically ideal lens anyway.) This would allow the photovoltaic panel to be much smaller than the lens and still capture energy over an area the size of the lens. It would *not* enable you to capture more energy per unit area of the earth's surface, but it would reduce the amount of photovoltaic material needed (assuming the small panels can handle the more concentrated energy input) and thus probably the cost of the system.
http://en.wikipedia.org/wiki/Lens_(optics)
Regarding the shape of the lens: any ideal lens should be a section of the surface of a sphere, but it should be possible to cut it so that the edges are in whatever shape you want. If you are thinking about an array of many lens+PV panel sets covering a field, then you might want to think about what shape could be repeated across the surface without leaving any gaps where no sunlight is collected. Regardless, if you are wondering what shape is best, this could be a question that you base your project idea on: decide what shape you think is best and why (hypothesis), then design and perform an experiment to test your hypothesis.
Some issues to worry about in a practical implementation: The lens would probably need to tilt to track the position of the sun throughout the day. PV panels may not operate as efficiently (collect as high a fraction of the input energy) when the input energy (in, say, watts/meter^2) becomes larger.
It actually looks like some similar ideas, if I understand your question correctly, has been put into production under the name "concentrator photovoltaics":
http://www.emcore.com/solar_photovoltai ... aic_arrays
http://www.physorg.com/news5214.html
http://www.parc.com/research/projects/c ... h/cpv.html
Amanda
this is because the sphere will concetrate the heat more, because it has no edges.