Solar panels now cost less to buy than they do to install.
Installing solar panels on roofs is significantly more expensive than putting them in fields. Solar panels have ended up on roofs of houses mostly because of subsidies, and partly because some people like the message that they send by having solar panels.
Putting solar panels over parking lots is also more expensive than putting them in fields, because people make holes in the asphalt for support poles and the poles need to be taller, but it's only a little bit more expensive and doesn't need extra land. Because shade over parking lots is a positive thing anyway, I think parking lots are a good place to put solar panels.
An important thing to note about solar panels is that because they have many cells in series, shading even a small part of them (eg with trees growing near houses) greatly reduces their power output.
With solar panel prices having come down so much, two-axis tracking is not cost-effective, but single-axis tracking is still generally worthwhile. 3x concentration of light with fresnel lenses could be viable, but the potential economic benefit would be fairly small, and highly concentrated photovoltaic power is highly impractical.
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